Previous to this invention, X-ray diffraction (XRD) measurements were made in a slow and tedious manner. FIG. 1 shows a schematic of the prior art, a typical X-ray diffraction apparatus. It comprises an X-ray source A, a collimator B, a specimen C, and an x-ray detector D. In the prior art the collimator is attached directly adjacent to the x-ray source. The collimator B changes the wide-angle output of the X-ray source A into a narrow beam. The x-ray beam E is aimed towards the specimen. The beam of X-rays E interacts with the specimen C. Some of the X-rays are diffracted by the specimen and are redirected towards the detector D. The X rays pass through the aperture F prior to entering the detector D. The characteristics of the diffracted X-rays are subject to mathematical description.
The Bragg Equation
The Bragg Equation: EQU n.lambda.=2d(sin .theta.) (1)
describes X-ray diffraction. Lambda, X, is the wavelength of the diffracted X rays in Angstroms. Theta, .theta., is 1/2 of the diffraction angle 2.theta., which is the angle between the incident and diffracted X rays. The crystalline lattice plane spacing is "d", in Angstroms. For non-crystalline materials "d" is the interatomic spacing. The order of diffraction is an integer, "n". For diffraction, "n" is never less than 1. It is well known in the art that for X-ray diffraction to occur, lambda, theta, and d must have the relationship described by the Bragg equation.
In the typical XRD apparatus, the x-ray source and its collimator remain fixed. The X-ray wavelength, .lambda., is limited to a single value. The point detector and/or the specimen are moved. The angle 2.theta., is thus measured allowing the detection of the various "d" spacings present in the specimen. Alternatively, 2.theta. may be held fixed and the wavelength, .lambda., varied. The prior art XRD apparatus may be utilized in one of the following XRD methods.
Powder Method
The XRD apparatus may be used with a powdered or polycrystalline specimen. It is assumed in the analysis that the crystallites of the powdered specimen are randomly oriented and that there are a great many crystallites illuminated by the incident beam. This allows the detector to be scanned in one dimension rather than two-dimensions. If the crystallites in the specimen are not randomly oriented, the emerging diffraction pattern will not be a series of uniform rings, but a constellation of spots or perhaps mottled rings. A linear scan would not correctly gather all the information contained in these mottled rings or spots. Specimen preparation for this type of analysis is time-consuming and requires skill and care.
When a specimen has few crystallites, or non-random orientation of its crystallites, the specimen can be rotated about one or more axes during the test. This simulates a specimen with random crystalline orientation distribution. However, the required movement often increases the time needed to perform the test. The data collected during a test where the specimen is rotated may be collected in a manner that preserves the rotation orientation information. The preferred crystalline orientation (also known as "texture") may be sensed using this technique.
Laue Equations and Method
The Laue method exploits the full, three-dimensional nature of X-ray diffraction. The Bragg equation is a simplification of the three-dimensional Laue Equations: EQU a1.multidot.(S-So)=h.lambda. (2) EQU a2.multidot.(S-So)=k.lambda. (3) EQU a3.multidot.(S-So)=l.lambda. (4)
X rays diffracted from a single incident beam, So, are diffracted in three-dimensions, not just in a single plane. The directions of the incident and diffracted beams are represented by So and S, respectively. The crystal lattice vectors are a1, a2, and a3. The Miller indices are h, k, and l. The variable .lambda. is wavelength, as it is for the Bragg equation, (1).
In the Laue method, the broad-spectrum ("white") X-ray source is collimated to a thin, pencil-like beam. A specimen, usually a single crystal, is placed in the path of the pencil x-ray beam. X rays are diffracted by the specimen and emerge in a variety of directions, as described by the Laue equations. A sheet of photographic film, or some other area detector, is placed near the specimen. The sheet of film may be placed behind the specimen, between the specimen and the X-ray source, or nearly anywhere as required by an operator and as dictated by the application. X rays that are diffracted by the specimen travel to the film and produce a "Laue pattern." If the specimen is a single crystal, the Laue pattern is a set of small spots. The film (or a typical position-sensitive area detector) does not record wavelength information, only intensity and location information. By knowing the relative location and orientation of the X-ray beam, the specimen, and the plane of the film, it is possible to accurately calculate 2.theta. for each of the spots.
Since there are usually a great many spots on the film, it is possible, by trial and error, to ascertain the crystalline orientation of the specimen, the crystal structure of the specimen and the dimensions of the crystal unit cell. However, it can be a tedious process.
Since film, and other position-sensitive area detectors cannot achieve high resolution for both position measurement and wavelength measurement, the Laue method is used principally on single crystals. This is because, without wavelength information, it is impossible to use the trial-and-error method if more than one crystal is illuminated by the X-ray beam. The lack of a practical means of gathering wavelength information as part of a Laue measurement severely limits the application of this method.
Area Detectors Vs Point Detectors
An "area" detector is sensitive to X rays in a plane, generally oriented normally to the diffracted X-ray beam. A line detector can also be scanned to function as an area detector. Typically, area detectors are not able to measure the wavelength of the X rays, only the intensity and position within the detector plane.
A "point" detector, as it is known in the art, is insensitive to position within its sensing region. The detector does not detect X-rays at an actual point, but over its volume. It does not distinguish where, within its volume, the X-ray photon was detected. Point detectors are often "energy-discriminating." That is, they produce a signal that can be processed to determine the wavelength of the detected X-ray photon.
Mono-chromatic vs. "Color" Detectors
An "area" X-ray detector may be substituted for the "point" X-ray detector described in the XRD methods above. An area detector is sensitive to X rays in a plane, generally oriented normally to the incident X-ray beam, rather than a single point. While point detectors can be made to detect both the wavelength and the intensity of X-rays, area detectors typically only sense the intensity. These detectors are mono-chromatic as to output signal and are known as "black and white" detectors. Area detectors have been produced that detect both X-ray intensity and wavelength. These are referred to as "color" area detectors. However, there is a fundamental compromise between spatial resolution and wavelength resolution. These "color" X-ray area detectors are very expensive.
If the orientation of a single crystal of a known substance is sought, a "black and white" area detector can be used. Since the "d" spacing of the substance is known and limited to a few values, the spots (of unknown wavelength) appearing on the area detector may be correlated to crystal lattice planes by a trial and error process. Thus, the orientation of the crystal may be determined.
The common thread of all of the prior art XRD apparatus is that the X-ray source is fixed in space and its output is collimated.
Recordation of Diffracted X-Rays
Recording the diffracted X rays in the prior art generally involves using photographic film to record the detected X-rays' images, regardless of their wavelength. However, use of photographic film for obtaining X-ray diffraction images such as Laue patterns has several disadvantages. The Laue pattern image is not immediately available because of the need to develop the film. Further, exposure time is prolonged as a majority of the X rays do not interact with the film. Fluoroscopic screens enable instant viewing of an image but have the disadvantage of requiring a darkened room for human eye viewing.
Efforts to resolve the problems associated with older X-ray imaging techniques have included use of an image intensifier and video imaging chain to generate a visible image on the screen of a display monitor. However, this produces a third generation image, which tends to be degraded by electronic noise. The first generation image appears on a fluorescent screen at the input of the image intensifier. The second generation image appears at another fluorescent screen at the output of the intensifier. The third generation image is produced by a video camera that views the image intensifier output. In order to improve image quality, the electronic signal generated by the image intensifier can be digitized to enable computerized image enhancement but this produces only marginal improvement.
In some more recent XRD systems, the image intensifier system is replaced with an array of minute electronic X-ray detectors such as charge coupled devices (CCD's). Data for constructing the image is read out of the CCD array on a pixel by pixel basis to provide an image, which may be displayed at the screen of a video display monitor. Of particular importance in utilizing a CCD array for acquiring X-ray diffraction imaging data is the small field of view of the CCD. The small field of view limits the angular range of the diffracted X rays the CCD is capable of detecting. Other position sensitive large area detectors suffer from lack of good spatial resolution and are very expensive.
Fundamental Differences between XRD and Radiography
In radiography, a shadowgraph, or radiograph, is made of the specimen by passing X rays through the specimen and detecting the directly transmitted rays that have not been absorbed or scattered by the specimen. The resultant shadowgraph resembles the physical shape of the specimen. The wavelength of the detected X rays is of little or no interest. As long as a large portion of the incident X rays are energetic enough to pass through the specimen, no thought is given to wavelength. X rays that do not take a direct path from the source to the detector are considered highly undesirable. A strong effort is made to prevent, or at least suppress, these nuisance rays from reaching the detector. The hardware in a radiographic system is specially fashioned and arranged to enhance the detection of directly transmitted primary rays and to strongly suppress detection of indirect, scattered, secondary rays.
An X-ray diffraction apparatus is quite different. The system components are fashioned and arranged to strongly suppress detection of the direct, primary radiation and to enhance the detection of indirect, secondary radiation. This is the complete opposite of a radiographic system. An XRD apparatus produces a pattern or a plot not a shadowgraph. The pattern or plot contains little or no information about the physical shape of the specimen. The pattern or plot instead reveals information about the atomic spacing and crystalline structure in a small region of the specimen.
The wavelength of the diffracted X rays is a key part of x-ray diffraction measurements. Most XRD systems restrict the source to a single wavelength, carefully sort the diffracted rays in terms of wavelength, or both. Only the traditional Laue method does not explicitly measure or restrict wavelength, but only single crystals can be analyzed using the traditional Laue method because wavelength information is not available. A tedious trial-and-error method must be used in lieu of wavelength information. If wavelength information could be recorded as part of a Laue measurement, it would no longer be restricted to use on single crystal specimens, but would have very broad application.
Devices representative of the art are:
U.S. Pat. No. 5,684,857 (1997) to De Bokx discloses a method for GE-XRF X-ray analysis of materials and apparatus for carrying out the method. FIG. 1 shows an X-ray source 4, a front collimator 14, specimen 16, back collimator 18, and detector 20.
U.S. Pat. No. 5,481,109 (1996) to Ninomiya et al. discloses a surface analysis method and apparatus for carrying out the same (see FIGS. 1, 7-16).
U.S. Pat. No. 5,457,727 (1995) to Frijlink discloses a device for processing a measured signal corresponding to the intensity of X rays reflected by a multi-layer structure on a substrate. FIG. 7 shows an X-ray source 1, a collimator system 2 and 3, a goniometer specimen support 9, a collimator system 5, and a detector 8.
U.S. Pat. No. 5,384,817 (1995) to Crowther et al. discloses an X-ray optical element and method for its manufacture. FIG. 1 shows an X-ray optical element and method for its manufacture. FIG. 1 also shows an X-ray source 12, a sample 16, a device 20 (which can be a collimator), a reflective element 22 and a detector 24. This figure is analogous to the typical arrangement as shown in FIG. 1 of the present application.
U.S. Pat. No. 5,267,296 (1993) to Albert discloses X-ray images produced on a monitor display screen by situating the subject between a detector having a minute X-ray-sensitive area and an X-ray source having an extensive anode plate on which an X-ray origin point is swept in a raster pattern similar to the raster of the display monitor.
U.S. Pat. No. 5,263,075 (1993) to McGann et al. discloses a high-annular resolution X-ray collimator. FIGS. 1-2 show an X-ray source 10, a slit collimator 20, and detectors 32.
U.S. Pat. No. 5,008,910 (1991) to Van Egeraat discloses an X-ray analysis apparatus comprising a sagittally curved analysis crystal. FIG. 1 of Van Egeraat shows a laser source 2, a specimen 6, an analysis crystal 8, a collimator 18, and a detector 16. This figure shows similar arrangement as shown in FIG. 1 of the present application.
U.S. Pat. No. 4,896,342 (1990) to Harding discloses an X-ray apparatus, which irradiates an examination zone in different positions by means of a primary beam having a small cross-section and a detector on the other side of the zone to measure the scattered primary beam.
U.S. Pat. No. 4,887,285 (1989) to Harding, et al discloses a method of determining the share of different chemical elements in an examination zone.
U.S. Pat. No. 4,850,002 (1989) to Harding et al discloses a two dimensional Compton profile imaging method and apparatus.
U.S. Pat. No. 4,104,519 (1978) to Oldendorf discloses a method and apparatus for retrieval of exposure information from film images. FIG. 5 shows a raster derive circuit 20, a source 12, a collimator 14, a filter 32, a film 16, and detector 26.
U.S. Pat. No. 3,949,229 (1976) to Albert discloses radiographic images of high definition and clarity produced quickly and with reduced radiation exposure of the subject by utilizing a scanning X-ray source in which a moving point source of X rays is created by sweeping an electron beam in a raster pattern on a broad anode.
U.S. Pat. No. 3,885,153 (1975) to Schoeborn et al. discloses a multi-layer monochromator. FIG. 2 shows two annular slits to produce a collimated neutron beam 13, a monochromator crystal 11, and a detector 15.
U.S. Pat. No. 3,373,286 (1968) to Han discloses a device for measuring the characteristics of a material moving on a conveyor with means for minimizing the effect of flutter. FIG. 1 shows a radiation source 2, a material 3, (which can be made of metal, plastic, etc. Column 3, lines 55-60), a collimator 12 and a detector 4. This patent shows similar arrangement as required in the reverse geometry embodiment of the FIG. 1 of the present application.
Recently, systems have been made available which provide for the movement of the X-ray source while the detector remains in a stationary position. These are unlike the foregoing fixed X-ray source systems. U.S. Pat. No. 3,949,229 ('229) to Albert is representative of such prior art. Albert '229 utilizes an X-ray generating component wherein a moving point source of divergent X rays is produced by scanning a broad area target plate with a charged particle beam. A relatively very small area radiation detector is spaced apart from the source to intercept X rays, which have passed through the subject undergoing examination. The output of the detector is used to control a cathode ray display tube or the like, having a raster pattern coordinated with that of the X-ray source, to produce a visual radiographic image of the subject. The detector output signals may also be stored on magnetic tape or by other means for later reconstruction as an image. Various electronic image enhancement techniques may readily be applied if desired. Automatic brightness control is provided in some forms of the Albert 229 invention to further reduce radiation dosage and to provide a more uniform contrast throughout different areas of the image by feeding back an average image intensity signal to the X-ray source to vary X-ray output in the course of the scanning as required for this purpose. Stereoscopic images may be produced by using two small area X-ray detectors which are spaced apart with each controlling separate visual images that are viewed by separate eyes of the observer or, in another form of the invention, by utilizing a single detector controlling the two separate images alternately wherein the raster pattern area at the X-ray source is alternately shifted between two at least partially separate areas of the target plate of the source.
Yet another invention representative of the art is U.S. Pat. No. 5,267,296 ('296) by Albert. In one aspect, Albert '296 provides X-ray imaging apparatus having an X-ray source which includes an anode plate, means for directing an electron beam to the plate to produce X-rays at an X-ray origin point on the plate, and means for traveling the X-ray origin point in a raster scanning motion within a first raster scan area on the plate in response to an x-axis sweep frequency signal and a y-axis sweep frequency signal. An X-ray detector produces a detector signal that is indicative of variations of X-ray intensity at a detection point that is spaced apart from the anode plate. A monitor has an image display screen and means for moving a visible light origin point in a raster scanning motion within a second raster scan area at the screen. The intensity of the light origin point is modulated during the course of the raster scanning motion at the second raster scan area by the variations of the detector signal which occur during the course of the raster scanning at the first raster scan area. The apparatus further includes means for producing a first sequence of digital data bytes which encode successive values indicative of variations in the magnitude of the x-sweep frequency signal that are to occur during the course of the raster scanning at the first raster area, means for producing a second sequence of digital data bytes which encode successive values indicative of variations in the magnitude of the y-sweep frequency signal that are to occur during the course of the raster scanning at the first raster area. Also included is means for producing the x-sweep frequency signal and the y-sweep frequency signal during the course of the raster scanning at the first raster scan area by conversion of the values encoded by successive data bytes of the first and second sequences into analog signals. Means are provided for producing and storing digital signals, which encode the location of a selected area of the image in response to area of interest selection controls. Further components include means for reducing the size of the first raster pattern at the anode plate in response to a zoom signal and means for positioning the reduced first raster pattern at a location on the anode that corresponds to the selected location on the image display screen that is encoded by the digital signals. Albert '296 provides a method for creating a radiographic image of a subject, which includes the step of scanning an electron beam in a first raster pattern on an anode plate to produce a moving X-ray origin point. X rays are detected at a detection point situated at the opposite side of the subject from the anode plate and a detector output voltage is produced in response to the detected X rays. Further steps include sweeping a light origin point on a display screen in a second raster pattern and varying the intensity of the light origin point at successive points in the second raster pattern in accordance with variations of the detector output voltage at corresponding points in the first raster pattern, selecting an area of the image at the display screen for magnification, encoding the location of the selected area in digital signals and initiating a zoom signal. Still further steps in the method include reducing the size of the first raster pattern in response to the zoom signal and positioning the reduced first raster pattern at a location on the anode plate that corresponds to the location in the image that is encoded in the digital signals. Albert '296 enables faster operation of reversed geometry scanning X-ray systems, simplifies the operator's control manipulations and expands the capabilities of the system with respect to producing images of different types by enabling digital data processor control of the scanning X-ray source and image characteristics. The operator may, for example, zoom in to magnify one or more areas of the image that are of particular interest by simple actuation of one or more standard computer input devices. High resolution scanning of the subject can be limited to selected regions, which are of interest, thereby reducing scanning time and minimizing radiation exposure of the subject. Magnified high definition images of selected regions of a subject can be acquired, stored, digitally enhanced in any of various ways and then be displayed sequentially or simultaneously. Albert '296 enables variation of the aspect ratio or height to width ratio of the image in response to digital signals to facilitate imaging of differently shaped subjects or, in the case of a moving subject, to compensate for an image distortion, which can otherwise result from the motion of the subject.